Infrared Camera Has Variety of Uses

A revolutionary new infrared camera developed by NASA scientists may soon present new possibilities to doctors, pilots and environmental scientists and as enable defense forces in the field to identify various types of rockets by their plumes.

The camera, developed at the Center for Space Microelectronics Technology at NASA's Jet Propulsion Laboratory (JPL), Pasadena, CA, in partnership with Amber, a Raytheon company, uses highly sensitive quantum-well infrared photodetectors, or QWIPS -- arrays of infrared detectors that cover longer wavelengths than are possible with previously existing detectors.

The camera is the only one of its kind at present, said the development team leader Dr. Sarath Gunapala of JPL.

The portable infrared video camera has 65,536 QWIPs arrayed in a focal plane of 256 by 256. Each QWIP is a pixel. The array is designed to detect infrared radiation in the 8-to 12- micrometer (millionths of a meter) wavelength range. This is 20 times longer in wavelength (lower in energy) than visible light. At these wavelengths room temperature objects naturally glow the same way red-hot objects glow in the visible.

"Infrared detectors operating in the 8- to 12-micrometer range have a variety of ground and space-based applications," said Gunapala. He said they can be used for early warning systems, navigation, flight control systems, weather monitoring and astronomy.

The higher sensitivity of long-wavelength QWIPs could allow doctors to detect tumors using thermographic, or heat, analysis, allow pilots to make better landings with improved night vision, and environmental scientists to monitor pollution and weather patterns.

Other possible uses include law enforcement, search and rescue and industrial process control, Gunapala said.

A quantum well, Gunapala said, can be imagined as a very small well with electrons in it in a state of rest. When they are disturbed by a photon, the smallest energy package in a beam of light, the electrons pop out.

Then the electrons produce a current which is proportionate to the amount of infrared photon energy which struck them. By measuring that current, the photodetector can tell how much infrared light comes from various sources at the scene being photographed.

Photodetectors, such as charged coupled devices (CCDs) in modern video cameras, "see" when the light has enough energy to knock electrons loose from the detector, creating an electric current. the longer the wavelength of light, the less energy the light has to give to the electrons and the colder the detector must be to avoid confusion with thermal vibrations. It is very difficult and expensive to make electronic materials that naturally have "loose" electrons, what may be called "low band- gap materials."

Quantum well photodetectors are another approach. In a QWIP detector, light is "seen" when it has the right energy (wavelength) to knock an electron out of the quantum well. Instead of looking for low band-gap alloys that have very "loose" electrons, quantum wells can be made-to-order for the wavelength wanted out of easy to manufacture materials.

For infrared light detectors to work, they must be very cold. The new camera, which weighs less than 10 pounds, contains a Stirling cooler, a closed-cycle refrigerator about the size of a fist. The small motor cycles the gas millions of times and cools the camera from room temperature to very low temperatures, about -343 degrees Fahrenheit (-208 Celsius), in about 10 minutes.

The camera can be hooked to batteries to make it more portable, but the current prototype plugs into a 110-volt wall socket for power. It is 4.4 inches wide, 10.3 inches deep and 7.2 inches high and the actual weight is 9.9 pounds.

This quantum well infrared photodetector technology has been developed over the past half decade under contract to NASA's Office of Space Access and Technology.